Support for reading 2.1 log files and upgrading from 2.1 store files

Length check is this: https://github.com/neo4j/neo4j/pull/2893/files#diff-2988a4fd11b2aee332d2a1fa0c10a7e8R60 right? I thought about ```"0.A.9".compareTo("0.A.10")```, but with length check it should be fine.